Hydraulic actuators are employed in a variety of applications to position a moveable member. For example, a variety of industrial applications, such as machine tools and robotics, include hydraulically actuated components. In addition, a variety of vehicles including submarines, helicopters and aircraft generally include a number of hydraulically actuated components. For example, modern military and commercial aircraft typically include a variety of control surfaces, such as ailerons, flaps and rudders, which are hydraulically actuated.
Consequently, a number of systems for controlling the hydraulic actuation of a moveable member have been developed. In order to increase the reliability of such hydraulically actuated control systems, a number of systems which provide redundant hydraulic actuation of a movable member have also developed. For example, control systems which employ redundant hydraulic actuation of a moveable member are described in U.S. Pat. No. 3,279,323 to C. W. Asche which issued Oct. 18, 1966; U.S. Pat. No. 3,338,139 to D. Wood which issued Aug. 29, 1967 and U.S. Pat. No. 4,436,018 to M. R. Murphy, et al. which issued Mar. 13, 1984, as described below.
The Asche '323 patent discloses a hydraulic actuator which is electrically controlled. The electrohydraulic actuator of the Asche '323 patent includes three independent signalling channels for transmitting signals, in parallel, from an input control terminal to a hydraulic motor. The three independent signaling channels each include an electrohydraulic flow control valve which controllably provides hydraulic fluid to the hydraulic motor in response to an input signal for driving a single output member. Fluid flow summing means is also in fluid circuit with the three parallel signaling channels to sum the hydraulic fluid flow provided by each of the three parallel channels to the hydraulic motor.
The electrohydraulic actuator of the Asche '323 patent also includes three transducers for generating negative feedback signals indicative of the position of the output member. According to the illustrated embodiment, each of the transducers is associated with a respective one of the independent signaling channels. A summing amplifier is also associated with each signaling channel to receive the negative feedback signal generated by the respective transducer and to add thereto the input signal. Accordingly, the fluid flow controllably provided by the hydraulic flow control valve of each of the signaling channels is adjusted in response to the position of the output member to reduce or eliminate the difference, if any, between the feedback signal indicative of the actual position of the output member and the input signal. According to the Asche '323 patent, the electrohydraulic actuator includes three parallel signaling channels and three associated transducers to provide redundancy such that the electrohydraulic actuator will continue to perform properly even if one of the signaling channels or one of the feedback loops malfunctions.
The Wood '139 patent also describes a redundant control system in which two or more servo control means are connected to each of a number of movable members. Sensor means, which generates an output signal in response to the physical position of an element within the system or a hydraulic or electric pressure within the system, are associated with each servo control means. The redundant control system of the Wood '139 patent also includes switch means, connected to each servo control means and operatively controlled by the sensor means, to deactivate a selected servo control means which has become inoperable. Due to the redundancy of the control system, however, the movable members can still be positioned by the remaining servo control means even though at least one servo control means has become inoperable.
In addition, the Murphy '018 patent describes a multiple loop control system which actuates a redundant tandem piston actuator. The redundant tandem piston actuator, in turn, provides a single composite output as evidenced by movement of a piston rod. The multiple loop control system of the Murphy '018 patent can include four identical electrical control loops which control four respective servo valves. The multiple loop control system also includes a failure management system for monitoring the electrical control loops and for disengaging control loops which fail. Due to the redundancy of the control system, the control system can perform appropriately even though one or more of the control loops may malfunction.
In a number of applications, hydraulic actuators are designed to meet two primary load requirements, namely, maximum output force and impedance or stiffness. As known to those skilled in the art, the output force F provided by a hydraulic actuator can be computed as the product of the hydraulic pressure P provided to the actuator and the hydraulic surface area A of the actuator. In other words, the output force F can be determined as: EQU F=P.multidot.A
Thus, by increasing the hydraulic pressure supplied to the actuator, the hydraulic surface area and, consequently, the physical size and weight of the hydraulic actuator can be decreased while providing the same output force to the control surface.
In addition to the decrease in size and weight of the hydraulic actuator, a reduction in the hydraulic surface area of a hydraulic actuator correspondingly reduces the hydraulic fluid flow requirements necessary to move the actuator at the same speed. Furthermore, by decreasing the hydraulic fluid flow requirements by increasing the hydraulic pressure of the system, the relative sizes of the pumps, hydraulic lines and hydraulic actuators can all be correspondingly reduced. Therefore, the overall size and weight of the hydraulic system can be further reduced. For example, an increase in the hydraulic pressure of a hydraulic system from 3,000 psi to 8,000 psi will generally reduce the hydraulic fluid flow requirements of the hydraulic system by greater than 50%, such as 62.5% in one exemplary hydraulic system, without reducing the speed of actuation of the hydraulic actuator.
As also known to those skilled in the art, the impedance or stiffness of a hydraulic actuator is the resistance of the actuator to an uncommanded deflection due to an external force. For example, actuators which position the various control surfaces of an aircraft can be deflected by external forces to which the control surfaces are subjected, such as air flow over the control surfaces. In order to prevent the external forces from deflecting or creating undesirable oscillations of the control surface, the hydraulic actuator must be sufficiently stiff to prevent deflections by external forces up to a predetermined maximum force.
Among other factors, the stiffness of a hydraulic actuator is dependent upon the size or hydraulic surface area of the hydraulic actuator. Accordingly, by decreasing the hydraulic surface area of a hydraulic actuator in order to decrease the size and weight of the actuator system, the stiffness of the actuator also generally decreases. Thus, while it is generally desirable to decrease the size of conventional hydraulic systems, such as the hydraulic systems employed on modern aircraft, in order to decrease the weight and fluid flow requirements of the hydraulic system, the impedance or stiffness of the resulting hydraulic actuators is correspondingly decreased. Thus, the control surface positioned by the hydraulic actuator may oscillate or be undesirably deflected by external forces, such as air flow thereabout, even though the hydraulic system pressure is increased.